FIG. 1 depicts a schematic diagram of base station 101 that comprises a single receive antenna, antenna 103, through which base station 101 receives signals transmitted from wireless terminal 107. As is well known to those skilled in the art, the use of a single receive antenna at base station 101 leaves the received signal quality vulnerable to a natural phenomenon known as Rayleigh fading.
Rayleigh fading occurs when the multipath components of a radiated signal destructively interfere at the receive antenna with the result that the signal-to-noise ratio of the composite received signal is below an acceptable threshold. Typically, a small change in the position of the wireless terminal changes the amplitude and relative phase of the respective multipath components at the receive antenna with the result that the signal-to-noise ratio of the received signal may be substantially improved. Unfortunately, another small change in position may degrade the signal-to-noise ratio. Empirically, as a wireless terminal moves and the path between the wireless terminal and base station changes with time, the received signal quality vacillates. This vacillation is known as Rayleigh fading.
To mitigate the effects of Rayleigh fading, base stations are often constructed with two, spatially diverse receive antennas. FIG. 2 depicts two antennas, receive antenna 203 and receive antenna 205, which are typically separated by some multiple (or submultiple) of the wavelength of the received signal's carrier signal. Because the signal path from wireless terminal 207 to each of receive antenna 203 and receive antenna 205 is always different, the multipath components at each receive antenna have different amplitudes and phases and, therefore, it is rare that the signal-to-noise ratio of each composite signal is simultaneously low at both receive antennas. Empirically, when the signal-to-noise ratio at one receive antenna is low, the signal quality at the other is typically satisfactory. This fact enables the base station to receive the transmitted signal at both antennas, to compare the relative signal quality at each antenna and to select, at each moment in time, the better signal. This technique is known as antenna receive diversity and it typically improves the overall received signal quality by 3 to 12 dB, or more.
Occasionally, the geographic area serviced by a base station is so large that the signals transmitted by a wireless terminal at the fringe of the area cannot be satisfactorily received, without assistance, by the base station. Alternatively, sometimes the geographic area serviced by a base station contains hills and valleys such that the signals transmitted by a wireless terminal to a base station are shadowed by the terrain. To alleviate this problem, wireless repeaters are employed.
FIG. 3 depicts a schematic diagram of base station 301, with receive antenna 303 and receive antenna 305, wireless terminal 307 and repeater 309, in the standard repeater configuration. The purpose of repeater 309 is to receive signals from wireless terminal 307, amplify them and re-transmit them to base station 301, when the distance or terrain makes it unlikely that wireless terminal 307 can transmit directly to base station 301.
Repeater 309 has one receive antenna for receiving signals from wireless terminal 307 and one transmit antenna for re-broadcasting signals those signals to base station 301. Because repeater 309 has only one receive antenna, the quality of the signals it receives are subject to Rayleigh fading. Furthermore, the fact that base station 301 incorporates receive antenna diversity becomes largely superfluous because the position of repeater 309 with respect to base station 301 does not change with time. Therefore, although repeater 309 is beneficial in that it boosts the overall signal strength of the signals from wireless terminal 307, the improvements in signal quality afforded by antenna receive diversity at base station 301 are lost.